Creep resistant superplastic alloys

ABSTRACT

THIS DISCLOSURE PROVIDES CREEP RESISTANT SUPERPLASTIC ALLOYS BY FABRICATING A PRIMARY SUPERPLASTIC ALLOY SYSTEM AND INCLUDING THEREWITH ALLOYING ADDITIONS WHICH DO NOT EFFECTIVELY ALTER THE SUPERPLASTIC PROPERTIES OF THE SYSTEM AT ELEVATED TEMPERATURES AND EFFECTIVELY REDUCE THE CREEP RATE AT LOWER OPERATIONAL TEMPERATURES BY PRECIPITATION OF THE ALLOYING ADDITIONS IN THE MATERIAL. ILLUSTRATIVE EXAMPLES OF ALLOYING ADDITIONS FOR THE ZN-AL SUPERPLASTIC ALLOY ARE THE ELEMENTS AG AND MN. THIS ALLOY HAS SUPERPLASTICITY NEAR 250*C. AND HAS ENHANCED CREEP RESISTANCE AT ROOM TEMPERATURE, E.G., 20*C.   D R A W I N G

May 28, 1974 P-. CHAUDHARI 1 4 CREE? RESISTANT SUPERPLASTIC ALLOYS Filed Dec. 5, 1971 2 Sheets-Sheet 1 FIG.1 (PRIQR ART)' FIG. 2 F A F A 0 A B c 15 1s 13 1s Fl G, 3 1 c 25 I INVENTOR 'PRAVEEN 01111110111111 POWER SOURCE ATTORNEY CREE? RESISTANT SUPERPLASTIC ALLOYS Filed Dec. 3, 1971 2 SheetS-Sheet 2 CONDITIONS FIG. 5A AT ROOM TEMPERATURE WIRE DIA. =o.051 m.

' cousmw LOA'D =6 LBS.

INITIAL STRESS=3000 LBS/SQ. IN. Zn AI MAGNITUDE 0F ERROR PERCENT ELONGAT'ON O Zn-AI +3.33 wr Ag A I I] n I Zn-A|+O.23 wT"/ ,Mn I '1 I I l o 48 es 444 492 240 288 33s 384 432 HOURS H 58 commons L6 7 AT ROOM TEMPERATURE WIRE DIA. =0.05I IN.

IA CONSTANT LOAD 12 LBS.

v INITIAL STRESS 6000 LBS./S0.IN.

1'2 MAGNITUDE 0F ER oR Zn-AI Zn --Al 1 0.23 w? Mn PERCENT .8

ELONGATION .2 Zn-Al+3.33 wf ,Ag 4 1 I I 1 4 1 I o 48 96144 492 240 zas 33s 3R4 432 nouns United States Patent 3,813,241 CREEP RESISTANT SUPERPLASTIC ALLOYS Praveen Chaudhari, Briarcliff Manor, N.Y., assignor to 1lzrltefrnational Business Machines Corporation, Armonk, Continuation-impart of abandoned application Ser. No.

837,822, June 30, 1969. This application Dec. 3, 1971,

Ser. No. 204,742

Int. Cl. C22c 17/00 US. Cl. 75-178 A 6 Claims ABSTRACT OF THE DISCLOSURE This disclosure provides creep resistant superplastic alloys by fabricating a primary superplastic alloy system and including therewith alloying additions which do not effectively alter the superplastic properties of the system at elevated temperatures and effectively reduce the creep rate at lower operational temperatures by precipitation of the alloying additions in the material. Illustrative examples of alloying additions for the Zn-Al superplastic alloy are the elements Ag and Mn. This alloy has superplasticity near 250 C. and has enhanced creep resistance at room temperature, e.g., 20 C.

This application is a continuation-in-part of copending patent application Ser. No. 837,822, filed June 30, 1969, now abandoned.

BACKGROUND OF THE INVENTION superplasticity is a condition whereby unusually large elongations occur in metals and alloys under uniaxial stress at moderately high rates of deformation. When a test specimen of a normal material is pulled in tension, it initially deforms uniformly but eventually some local instability causes the specimen to neck down, i.e., there is a local change in cross-sectional area. This is normally the beginning of fracture of the specimen, since the stress at the neck by virtue of the reduced cross-section is larger than the stress in the rest of the specimen. As a result, most of the deformation is concentrated at the neck where the specimen eventually fractures. In contrast, a change in stress in a superplastic material does not cause a very large change in strain rate, i.e., rate of deformation. Therefore, the area around the neck deforms at a rate comparable to the rest of the specimen. The small sensitivity to necking is one of the more distinctive characteristics of superplastic materials. Regions of high stress do not become highly strained as in normal materials. Illustratively, a sheet of superplastic material if pulled over a sharp edge does not thin out and break as does a normal material in the same circumstances, but it has a tendency to redistribute some of that strain to other parts of the material. Similarly, a thin spot in a stressed superplastic material does not automatically get thinner when the entire material is deformed, but it thins out at a rate comparable to the thinning in the rest of the piece of the material.

One of the limitations on the use of superplastic materials in the prior art has been their tendency to creep at normal device operational temperatures, e.g., room temperature of 20 C. The phenomenon of creep results in deformation of a piece of material under stress, i.e., under mechanical loading conditions over a related significant time interval. The consequence of the creep phenomenonis that the shape of a fabricated piece becomes considerably distorted and usefulness of the piece may become severely impaired as a consequence of the resultant deformation over a time interval.

According to the postulate of grain-boundary sliding, flow of a polycrystalline material occurs through relative 3,813,241 Patented May 28, 1974 motion between grains. As the grains are irregular and interlock with one another, only a limited amount of flow can occur Without an additional mechanism that allows the grains also to deform. Sliding of the grains relative to each other provides more degrees of freedom than are present if a grain boundary cannot slide. Superplastic materials are temperature-sensitive partially because of the effect of temperature on sliding. Illustratively, a grain boundary can be regarded as a layer of liquid-like material; i.e., an increase in the temperature decreases the viscosity or increases the fluidity of the grain boundaries.

The terminology grain boundary includes grain boundaries found in single phase superplastic alloy, and interphase boundaries in multiphase superplastic alloys. An interphase boundary is a boundary between two phases which may, for example, differ in composition or crystal structure.

It has been demonstrated in the prior art both theoretically and experimentally that dislocation motion is the dominant mechanism in the deformation of most materials. A dislocation is a line defect in a crystalline solid that allows crystal planes to slip over each other a little at a time. An analogy for the relative motion of two crystal planes by dislocation motion is the wrinkle that is purposely put into a rug for adjusting its position on a floor. It takes a great deal less mechanical effort to put a wrinkle in one side of a rug and move the Wrinkle across the rug than it takes to move the rug the same distance all at once. The presence and movement of such dislocations in metals on millions of parallel slip planes leads to macroscopically homogeneous deformations.

Diffusion creep or Nabarro-Herring creep is a mechanism that accounts for crystalline flow by the diffusion of atoms under a concentration gradient of vacancies. A vacancy in a metal is a void region where normally an atom would be expected. The concentration gradient of vacancies behaves in such a way that atoms diffuse from a surfaceunder compressive stress to a surface under tensile stress. In polycrystalline materials these surfaces under stress correspond to grain boundaries. Since it takes a shorter time for vacancies to diffuse through small crystals than through large crystals, diffusion creep can account for the very large elongations in superplastic alloys at the rates observed.

A prior art procedure described in an article by P. C-haudhari in IBM Technical Disclosure Bulletin, Vol. 10, No. 6, November 1967, page 687, provides a heat treatment which lowers the high creep rate by factors of 10 to 10 without substantially affecting the superplastic properties of the Zn-Al alloy. The Zn-Al alloy eutectoid composition, 22 wt. percent Al 78 wt. percent Zn exhibits superplasticity when cooled rapidly from above the invariant temperature of 275 C. 'Eutectoid transformation are solid state transformations where a single phase 7 decomposed to two phases a-I-B. The Zn-Al alloy system has optimum superplastic properties at compositions of approximately the eutectoid composition. A product manufactured using superplastic properties of this alloy has a relatively high creep rate at room and elevated temperature. After the product is manufactured utilizing the superplastic properties, it is heated to above the invariant temperature of 275 C., where it is held until the two phases n+5 transform to the 'y-phase. On completion of this transformation, the product is placed in a furnace maintained at a temperature between C. and the invariant temperature until the -phase transforms to the two phases oc-l-fl. The creep rate is decreased when the alloy is returned to ambient temperature fromthat creep rate it had prior to the noted treatment.

Although this prior art heat treatment for improving creep resistance of superplastic alloys is beneficial for many superplastic alloys, and is highly important for the Zn-Al superplastic alloys system, it does not impart the totally requisite creep resistance.

A background article of general interest on superplasticity is presented in the Journal of Science and Technology, September 1968 by P. Chaudhari.

OBJECTS OF THE INVENTION It is an object of this invention to provide a superplastic alloy with enhanced creep resistance at operational temperatures for the alloy.

It is another object of this invention to provide a creep resistant superplastic alloy which derives its creep resistance from precipitation at grain boundaries.

It is another object of this invention to provide a creep resistant superplastic alloy of the eutectoid Zn-Al system by including therein solute additions, e.g., Ag and Mn singly or collectively.

It is an object of this invention to provide creep resistant superplastic alloys by including therein additional elements to form precipitates at ambient temperatures which block the movement of defects or attach themselves to the defects thereby slowing them down.

It is another object of this invention to provide creep resistant superplastic alloys by including therein elements which are soluble at high temperatures and are less soluble at ambient temperature so that the elements precipitate out of solution on suitably cooling of the superplastic alloy.

It is another object of this invention to provide creep resistant superplastic alloys by controlling a precipitation reaction by suitable heat treatment which favors precipitation of solute elements in the superplastic alloy along grain boundaries and in grains such as to impede the movement of defects such as dislocations.

SUMMARY OF THE INVENTION Through the practice of this invention the creep behavior of superplastic alloys is improved at operational temperature. Precipitation hardening is achieved by precipitation of solute addition at grain boundaries to decrease their mobility and in the grains to pin defect movement. Generally, for a superplastic alloy of two components, a third component is selected which goes at least partially into solution at a temperature below the invariant temperature of the superplastic alloy system. The alloy system with the additional component is prepared in its superplastic form and deformed. After deformation, the piece is cooled rapidly to a temperature when precipitation caused by the additional component occurs in the matrix and at grain boundaries. Illustratively, the creep rate at ambient operational temperature, e.g., 20 C., of the Zn-Al (zinc-aluminum) eutectoid alloy system is improved if A-g (silver) or Mn (manganese) is added to the alloy system.

The additions may be selected by inspection of the phase diagram for the superplastic alloy and the solid or solids to be added. The criteria involved are that the element added should be in solution at the temperature where the material is to be deformed superplastically and precipitate at the ambient temperature where creep resistance is required. Illustratively, the elements with suitable properties for practice of this invention can be selected from the text Constitution of Binary Alloys, by M. Hansen et al., McGraw-Hill, Inc., 1958 which provides background material on binary phase diagrams. Once the elements have been selected that go into solution in one or preferably all the phases that may be present in a superplastic alloy, the next requirement is to precipitate them at grain boundaries. In order to do this, it is required that the driving force for precipitation reaction be effectively small. Precipitation at grain boundaries can be determined by microscopic examination, and for superplastic alloys, this can preferably be accomplished using a high power microscope such as the electron-microscope. The solid elements should preferably precipitate forming compounds because compounds usually have high temperatures of melting and show accompanying properties such as low diffusion rates and high hardness. The precipitate effectively tends to block the movement of the defects, such as grain boundaries. Where the solid element does not precipitate out as a compound, it is required that the solid element should be harder and have a lower diffusion rate than the superplastic alloy. Further, a desirable property of the precipitate is that it strain the matrix when formed or nucleated. Such a strain field tends to minimize defect movement and favors precipitation at the grain boundaries. Solid elements that favor precipitation at grain boundaries by virtue of the strain field can be selected by examining the lattice parameter of the matrix and the lattice parameter of the precipitate. In order to get a precipitate that introduces or generates a strain in the matrix, the lattice parameter differential between the precipitate and matrix should not be too small. In selecting solute elements for the practice of this invention background information may be obtained from the text Precipitation Hardening by A. Kelly and R. B. Nicholson, Progress in Material Science, Volume 10, Editor B. Chalmas, Pergamon Press, New York (1963).

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 presents schematic diagrams FIGS. 1A, 1B and 10 which illustrate the deformation characteristic of a normal material under loading conditions in which FIG. 1A illustrates the initial condition of the fabricated part. FIG. 1B illustrates the onset of necking and FIG. 10 illustrates fracture of the fabricated parts.

FIG. 2 presents schematic diagrams FIGS. 2A, 2B, 2C and 2D which illustrate the deformation characteristic of a superplastic material under loading condition in which:

FIGS. 2A, 2B and 2C illustrate superplastic deformations with relatively different elongations were FIGS. 2A and 2B show the early stages of deformation and FIG. 2C shows the fabricated part in a superplastic condition; and

FIG. 2D illustrates a fabricated part after superplastic deformation in a condition where its creep resistance has been improved.

FIG. 3 presents schematic diagrams which illustrate the effect of precipitates in grain boundaries and in grains of a superplastic alloy on grain boundary sliding.

FIG 4 is a schematic diagram which illustrates a technique for introducing solute additions into superplastic material.

FIGS. 5A and 5B present for different loading conditions test data on creep rate at ambient room temperature for superplastic alloys Zn-Al, Zn-Al-i-Ag, and

zn-Al-i-Mn.

EMBODIMENTS OF THE INVENTION FIGS. 1 and 2 provide schematic illustrations of testing procedures for differentiating between the properties of normal and superplastic materials under uniaxial mechanical stress. Tensile tests are conventionally carried out by clamping a cylindrical test specimen 10 at one end 12 to mount 13 and loading it at the other end 14. The actual operational setup for performing a tensile test can vary widely since the nature of the specimen and the test environment must be accommodated. In FIG. 1 A there is shown the test speciment 10-1 before significant necking has occurred. FIG. 1B shows the onset of necking 161, and FIG. 1C shows the fracture 18-1 which has occurred as a consequence of necking. FIGS. 2, 2A, 2B and 2C illustrate the comparable test experiments for a superplastic material in accordance with this invention. In FIG. 2A the material -2 is shown in its substantially unelongated position. FIG. 2B indicates the deformation under comparable loading conditions for the normal material of FIG. 1B, and FIG. 2C shows the superplastic material in an extended position for comparison with the normal material of FIG. 1C.

FIG. 2D is a schematic diagram of a fabricated part 19-1 in a condition in which creep is a significant factor. The fabricated part 19-1 is comparable to the superplastic alloy 18-2 of FIG. 2C which has been cooled after having been deformed to the elongation shown. Fabricated part 19-1 is supported via coupler 19-2 from supporting mount 19-3. Weight 194 is connected to fabricated part 19-1 at its lower end via connector 19-5 and coupler 19-6. Because the fabricated part 19-1 has an alloying condition therein according to the principles of this invention, it is resistant to creep at the operational temperature, e.g., ambient temperature 20 C. However, load 194 would cause a significant increase in length of fabricated part 19-1 at a relatively high temperature. Illustratively, fabricated part 19-1 is a Zn-Al alloy with solute addition Ag therein which would be in solution were the part to be raised to a relatively high temperature but which has precipitated at preferential nucleation sites at the relatively low temperature of the creep test.

v,FIG. 3. presents schematic and idealized drawings for illustrating the effect of grain boundary sliding and the pinning of the grain boundary sliding by solute additions in accordance with this invention. FIGS. 3A and 3B show two idealized two-dimensional hexagonal grains 20 and 22 of a conventional superplastic alloy, e.g., Zn-Al, before and after creep under a given loading has occurred at grain boundary 24 under a mechanical stress 25.

FIGS. 3C and 3D illustrate the presence of a solute addition 26 in the grain boundary 24 and the indication that the grain boundary sliding has not occurred as much for comparable mechanical stress conditions as imparted to the grain boundary to obtain the grain boundary sliding of FIG. 3B. The solute addition in the grain boundary acts to impair the grain boundary sliding but usually can'- not totally overcome it. Dislocations 28 shown in FIG. 3D may be generated at the junction 30 of the grains 20 and 22 where the grain boundary sliding is manifested. Dislocations are shown in FIG. 3D being generated at the junction between the grains. These dislocations for the circumstance of FIG. 3B may propagate across the adjacent grain 32 and cause the grain boundary 24 which has been partially pinned by the solute addition 26 to move a little and under continued mechanical stress cause the total grain boundary sliding as if the pinning precipitate were not there. However, the time within which the total grain boundary sliding occurs is greater because the rate of sliding is less due to the pinning precipitate in the grain boundary. With reference to FIG. 3B, by introducing precipitates into grains, e.g., 34, the creep rate of a superplastic alloy at ambient temperature is improved by the practice of this invention. The preparation of Zn-Al superplastic alloys can be carried out by melting Zn and Al in a suitably inert crucible. Similarly, a Zn-Al alloy containing Mn, Ag and other elements collectively can be prepared. In certain cases, for preparation of a superplastic alloy of this invention, it is desirable to obtain an alloy of Mn or Ag with Zn and Al and use them rather than pure Mn and Ag which avoids undesirable high temperatures of melts.

FIG. 4 is a schematic diagram showing apparatus for preparing composite superplastic alloys for the practice of this invention. A crucible 40 of graphite or tantalum is used for the Zn-Al alloy. The additional Ag or Ag-l-Mn is added to the crucible and the temperature is raisedabove 500 C. until the Zn-Al is in the liquid state and the'solute addition dissolves into solution. The crucible 40 can be heated either by an external heat source, not shown, or by an electrical current imparted to the resistive winding 42 from a power source 44. After the solute addition dissolves into solution, the material in the crucible is cooled either by pouring it into a separate mold which is cooled or by quenching the crucible 40 itself. Thereafter, a useful workpiece of the superplastic material is fabricated by conventional procedure. A background article on the fabrication of work-piece of superplastic material is IBM Jaurnal of Research and Development, Volume 9, p. 134 (1965) by D. S. Fields, Jr.

' PRACTICE OF THE INVENTION TheZn-Al eutectoid system in the superplastic state consists of two phases, i.e., a Zn-rich phase and an A1- rich phase. The alloy is obtained in the superplastic state by rapid cooling and subsequent decomposition into the two phasesat approximately ambient temperature and below, e.g., 20 C. Although the Zn-Al eutectoid system can be established as a superplastic material at about 250 C., it has a high creep rate at room temperature. It has been discovered for the practice of this invention that suitable alloying additions, e.g., Ag and Mn, have only a small effect on the superplastic microstructure. By controlling the composition of the Zn-Al alloy, Ag or Mn go into solution at a temperature where superplasticity occurs and do not strongly influence the superplasticity. At room temperature where Ag and Mn occur as precipitates, creep resistance is enhanced.

In the practice of this invention, for the Zn-Al alloy system, an element is selected that goes into solution at 250 C. but does not do so at room temperature where the improved creep resistance is desired. Ag goes into solution at 250 C. in Al up to 1.5 wt. percent and in Zn up to 3 wt. percent. However, at room temperature the solubility of Ag and Al is negligible and the solubility of Ag in Zn is comparably small. When approximately 2 to 3 wt. percent Ag is added to the Zn-Al eutectoid alloy system at the superplastic temperature, no significant deleterious mechanical defects are encountered. However, at room temperature, e.g., 20 C., the Ag precipitates out of solution and the Zn-Al alloy system at that temperature is creep resistant. An illustrative technique for making work-piece parts is shown by introducing Ag into Zn-Al eutectoid alloy system. This may be done by adding approximately 2 to 4 wt. percent Ag to a eutectoid melt of Zn-Al, or mixing the Ag with Zn-Al and melting the same. The melt is cooled from about 300 C. to form a superplastic microstructure which deforms superplastically at 250 C. After the deformation and forming of the work-piece part into a desired shape, it is heated to 350 C. and held at that temperature until transformation of the alloy to the 'y-phase is completed. Thereafter, the material is cooled to below 275 C. but above 250 C. until the material transforms into the two-phase structure of a-phase+p-phase. Then the work-piece part is quenched to room temperature and then aged by additional heat treatment at C. to precipitate out Ag. The additional heat treatment is usually required for those elements which have slow kinetics, i.e., they are sluggish at room temperature. If the kinetics are sufficiently rapid at room temperature, slow cooling to room temperature is often sufiicient to cause the desired precipitation. In some circumstances, furnace cooling is often sufiicient. The function of the heat treatment is to precipitate out Ag at selected boundaries which include the interface boundaries between the two phases, the a-phase and ,B-phase, and also other sliding boundaries associated withthe junction of colonies in the two-phase structure.

The following discussion presents actual data as illustrated in FIGS. 5A and 5B on the improvement of creep resistance of superplastic alloys prepared in accordance with the principles of this invention. Specimens of a Zn-Al alloy, a Zn-Al alloy containing 0.23 wt. percent Mn and Zn-Al alloy containing 3.33 wt. percent Ag were prepared in an identical manner into circular wire shapes of original diameter 0.051 in. and tested for creep at room temperature under different constant loads corresponding to initial stress of 3 l0 and 6 10 p.s.i. The percent elongation which is the measure of creep, is shown plotted against time in hours in FIGS. A and 5B. It is noted that for a given time the percent elongation is always smaller for Zn-Al with an alloying addition which creep resistance had improved under the same stress conditions for a superplastic Zn-Al wire with Ag or Mn compared with a superplastic Zn-Al wire without solute addition. In greater detail, the measurements of creep were made by determining the distance between two scribed lines on the test specimen. Because of errors in measurement of i 0.001 cm. due to the technique of marking a specimen, with the noted scribed lines, an error of i 0.03 percent occurred in measurement of the elongation. The maximum magnitude of error is shown on FIGS. 5A and 5B. The magnitude of error indicates the estimated possible uncertainty in an elongation measurement taking into account the actual circumstances of the creep test. Three consecutive measurements were taken approximately every 24 hours and the average was used for determining deformation and percent elongation for every point shown in FIGS. 5A and 5B.

The following Table I presents an exemplary list of superplastic material whose creep resistance can be improved at operational temperature by solute addition in accordance with the principles of this invention:

TABLE I Materials (percent Temperature where mateby weight): rial is superplastic C.) Aluminum-Copper (67-33) 500 Aluminum-Silicon (88.3-11.7) 550 Aluminum-Zinc (Several compositions) 250 and up. Bismuth-Tin (Several compositions) 30 Bismuth-Lead (56.5-43.5) 30 Chromium-Cobalt (72.5-27.5) 1200 Copper-Magnesium (81.3-19.7 650 Iron-Chromium-Nickel-Titanium Aluminum- Carbon (68.49-25-5.7-0.69-0.1-0.02) 925 Iron-Manganese-Carbon (97.68-1.90.42) 750 (Steels of various composition) 800-900 Magnesium-Aluminum (67.7-32.3) 375 Magnesium-Copper (69.3-30.7) 450 Magnesium-Nickel (76.5-23.5) 475 Nickel Chromium-Iron-Titanium Aluminum (48.25-39-10-L75-1) 900 Lead-Cadmium (82.6-17.4) 100 Lead-Tip (Several compositions) 30 Titanium-Aluminum-Vanadium (90-6-4) 950 Titanium-Aluminum-Tin (92.5-5-2.5) 1000 Zircalloy-4 900 THEORY OF THE INVENTION Plastic flow must be minimized in order to improve the creep resistance of superplastic alloys. There are three main mechanisms of plastic deformation, i.e., diffusion creep, boundary sliding, and dislocation motion. All three mechanisms have been determined to operate to some extent in superplastic alloys. Ways of inhibiting grain boundary sliding and dislocation motion have been determined for the practice of this invention, and barriers to the motion of the defects are introduced into a superplastic alloy. The barriers are introduced by precipitation of alloying additions included in the primary superplastic system. The precipitates manifest obstacles which inhibit grain boundary sliding movement and also inhibit motion of dislocations which may accompany grain boundary sliding. For the practice of this invention, an alloying addition should exhibit the following properties:

It should go into solution in the primary alloys at the temperature range where superplastic properties are of 8 interest; it should not inhibit the formation of fine microstructure which is a pre-requisite to obtaining superplasticity; and it should form hard precipitates with low dif* fusion rate, at temperature where creep resistance is important.

In thermodynamic terms, as temperature of a superplastic alloy is lowered, the solute tends to precipitate out of solution. As a certain amount of energy is required for the nucleation of precipitates, the amount of nucleation varies with the nature of the nucleation site. Most grain boundaries have lower energies for nucleation than the bulk material and this lower energy preferentially favors nucleation at a grain at boundary. Another factor is the driving force for precipitation. The solute has enough energy available to precipitate in a variety of places for a very high driving force, but if the driving force is small, the solute precipitates at preferential sites, i.e., grain boundaries and other inhomogeneities in the superplastic material.

There are two or more phases present in some superplastic systems which are ductile and have comparable fractional volumes. In the practice of this invention, it is desirable for such superplastic systems to use an alloying addition which hardens both of the phases at the operating temperatures where creep resistance is desired but does not harden the phases at the superplastic temperature range. Further, a plurality of elements may be selected as alloying additions; each element is selected for hardening of a particular phase. Further, the individual elements should not have deleterious effects on the superplastic property unless the effect of one element is compensated by the addition of another element.

Precipitation hardening for the practice of this invention uses the presence of precipitates in a material to provide obstruction to the movement of defects therein. The precipitate tends to harden the material. Firstly, the precipitate introduces an elastic strain in the lattice which then interacts with the defects and pins them down. Secondly, the precipitate provides a physical obstruction to the defect so that the defect has either to detour around the precipitate or pass through it. A background book on precipitation hardening is, Dislocations, J. Friedel, Addison Wesley Publishing Co., Pergamon Press, 1967, pages 368 to 414.

In the practice of this invention, solid elements are se lected as solute additions for a superplastic alloy which are in solution at relatively high temperature and which precipitate at ambient temperature. Superplasticity is present in materials which have a very fine grain size. These grain sizes can be obtained by a variety of techniques. Two of these techniques are eutectic decomposition and eutectoid decomposition. In eutectic decomposition, the material transforms from the liquid state to two-phase solid state. In the eutectoid decomposition, the material transforms from a solid state to two other solid phases. Once the fine grain size has been obtained, the material is superplastic at temperatures where grain boundary sliding becomes important. In some superplastic alloys, the grain boundary sliding can occur at room temperature or operating temperature which leads to high creep rates. This invention provides a technique whereby the high creep rate is minimized by inhibiting grain boundary sliding and other defect movement which accompanies grain boundary sliding. 7

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

-' What is claimed is:

1. A method of forming creep resistant work-piece parts from superplastic alloys comprising the steps of:

(a) heating a mixture of an alloy of Zn-Al wherein said Zn is present in about 78% by weight and the remainder is Al and an elemental metal selected from the group consisting of Ag and Mn in an amount of about 2% by weight to about 4% by Weight to a temperature sufficiently high to dissolve said metal into said alloy;

(b) cooling said alloy .to its superplastic state;

(0) deforming said superplastic alloy, thereby forming a work-piece part in a desired shape;

(d) quenching said alloy to a temperature below its superplastic deformation temperature, and thereafter;

(e) aging said alloy by heating the same above said quenching temperature but below its superplastic deformation temperature to precipitate said elemental metal therein thereby establishing said creep resistance in said work-piece part.

2. A method according to claim 1 wherein said melt is cooled to about 250 C. in step (b).

3. A method according to claim 1 wherein said deformed superplastic alloy is heated to about 350 C. prior to step (d) but after step (c). v

4. A method according to claim 1 wherein said elemental metal as Ag and is present in the amount of about 2% by weight to about-4% by weight.

5. A method according to claim 1 wherein said elemental metal is Mn and is present in the amount of about 2% by weight to about 4% by weight.

6. A work-piece of snperplastic material characterized by exhibiting substantial creep resistance comprising a mixing of a superplastic alloy of Zn-Al with approximately 78% Zn by weight and an elemental metal selected from the group consisting of Ag and Mn in an amount of about 2% by weight to about 4% by weight.

References Cited UNITED STATES PATENTS 3,519,419 7/1970 Gibson et a]. 148- 12] 3,676,115 7/1972 Hare et al. 148-11'.5 R 3,420,717 1/1969 Fields et a1. 148-115 R FOREIGN PATENTS 1,192,945 5/1970 Great Britain.

RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. 14812.7, 32.5 

